Thats probably the fluid spraying on something hot - it makes a smell like burning plastic.

Thank you...yes it smelt like burning plastic..... in fact thank you all for the replies and the info from previous write ups. Armed with this I have been outside since sun up, investigating further.

It is impossible to accurately establish the specific source of the pressurised leak so I am thinking I should replace the S shaped return pipe and start there.
The part of it that I can see is quite 'weathered' so it is not inconceivable to believe it could be worse further along.

One further question. Is it possible to replace said pipe with the engine under-tray in place?

I think I see the confusion. Delboy's helpful photographic guide is for replacing item 18 (S shaped pipe) but others have experienced issue with and also replaced item 10 correct?
The item number on Delboys list is for item 10 though......

I'd stripped the car down as described in Delboys guide already and today found a small tear in the hose underneath the shrouding, which I had to pull back to access.

I took a different approach. I assured myself that I had found the culprit section and cut the hose 1 inch above the tear. I had bought a section of hydraulic hose already, cut it to length and joined it to the existing hose.

Re-assembled using jubilee clips, filled and bled the system, the grinding & noises slowly reduced and all seems fine. It was a right pain taking the airbox apart again just to check the level after a drive around town, really annoying isn't it!

I'll keep an eye on it and hope my repair works. Oh and I need to buy lots more screws to better reattach the wheel arch liner.